A lens is one or more pieces of optical material (most commonly glass or plastic) designed to collect and focus all wavelengths of light at a single focal point to form a sharp image on the film.

When you look at the specifications of a camera you will notice that its lens is made of elements and groups.

The term element refers to the individual
pieces of glass. The term group can refer to a single element, or two or more elements
cemented together.
So a lens that has 5 elements in 4 groups has 5 pieces of glass with 2 cemented together.

The use of multiple elements allows more optical aberrations to be corrected than is possible with a single element, but by itself the number of elements is no guarantee of quality.